The invention relates to an ablation catheter which controls the temperature and reduces the coagulation of biological fluids on a tip of a catheter, prevents the impedance rise of tissue in contact with the catheter tip, and maximizes the energy transfer to the tissue, thereby allowing an increase in the lesion size produced by the ablation. The ablation catheter includes a tip for applying electrical energy to biological tissue. Passages are positioned within the tip in a variety of manners for directing a fluid flow through the tip to the exterior surface of the tip to control the temperature and form a protective fluid layer around the tip. Monitoring structure is also positioned within the tip for measurement of the electrical potentials in a biological tissue. Ablation electrode structure is also positioned within the tip for application of ablative energy to the biological tissue. A flexible, extended embodiment electrode provides the capability to form deep, linear lesions along a portion of a heart wall during ablation for the treatment of particular arrhythmias.